SystemModel: update_overall_statistics() cache invalidation and refactoring

[lyuyangh]

part of #433

• removes stale analysis_cases from storage if update_overall_statistics() is called multiple times. This method can actually be called multiple times now. (fixes remaining issues from implement update_overall_statistics() and refactor create_system #483 )
• update_overall_statistics._process() relies on metadata. This does not work because metadata is only available when the system is submitted (metadata is not available when we call update_overall_statistics() to update the cache). This PR is a first attempt to remove the dependency on metadata. To remove the dependency completely, some additional fields need to be stored in the DB. I will follow up with another PR to handle those fields.
• get_private_properties() now raises a ValueError if the system doesn't exist instead of aborting. If the system doesn't exist when this method is called, it indicates a misuse of the method so an Exception is more appropriate.

Compatibility

• No DB migration required.
• Does not change the public interface so explainaboard_client shouldn't be affected.